1. Field of the Invention
This invention relates to an optical displacement measurement device and an optical displacement measurement system for irradiating an object with beam light and detecting reflected light from the object, thereby detecting the distance to the object and displacement from the reference position of the object by triangulation.
2. Related Art
Hitherto, an optical displacement measurement device has been known which irradiates an object 3 with beam light provided by passing infrared light emitted from a laser diode 11 through a light transmission lens and receiving diffused reflected light from the object 3 at a position detection element 21 through a light reception lens 22 of a light reception optical system, thereby finding the distance to the object 3 (or displacement from the reference position of the object 3) by applying the triangulation principle, as shown in FIG. 29. That is, the image of a light transmission spot formed on the surface of the object 3 by irradiating the object 3 with beam light is formed on the light reception face of the position detection element 21 through the light reception lens 22 for forming a light reception spot and as the distance to the object 3 changes, the light reception spot formation position changes, the fact of which is used to find the distance to the object 3.
The laser diode 11 is driven by a drive signal having frequency fA output by an oscillator 13 and passed through an LD drive circuit and outputs laser light modulated so that light output is changed like a sine wave.
The position detection element 21 uses a PSD (position sensitive device) placed so as to match the length direction with the light reception spot move direction or two diodes placed in the light reception spot move direction. The PSD, which is a semiconductor device having a pin structure, comprises a pair of electrodes provided at both ends of the length direction of the light reception face and a common electrode. When a light spot is formed on the light reception face, resistance between the electrodes at both ends is divided at the light spot position in response to the light spot position. That is, a constant current is supplied from the common electrode, whereby position signals 11A and 12A having a current value at the ratio responsive to the light spot position are output from the electrodes at both ends. Since the light spot position is relative to the ratio between the position signals 11A and 12A, the position of the light spot formed on the reception face of the PSD becomes a function of (11A-12A)/(11A+12A) or corrected value thereof.
The position signals 11A and 12A of electric signals output by the position detection element 21 are converted into voltage signals by I/V conversion circuits 23a and 23b and the voltage signals are amplified by amplifiers 24a and 24b, then synchronously detected by detection circuits 25a and 25b, whereby only signal components Vd1A and Vd2A are extracted. The detection circuits 25a and 25b are controlled in detection timing by a timing signal generated by a timing circuit 28 based on output of the oscillator 13. Since the signal components Vd1A and Vd2A thus extracted are like pulsation waveforms (as the laser light is modulated), position information signals V1A and V2A provided by averaging output values of the detection circuits 25a and 25b through low-pass filters 26a and 26b are found to extract the signal levels.
Since the position information signals V1A and V2A are electric signals having signal values proportional to the signal values of the position signals 11A and 12A, if an operation section 27 finds (V1A-V2A)/(V1A+V2A), information equivalent to the distance to the object 3 can be provided. That is, the operation section 27 consists of a difference calculation section 27a for finding (V1A-V2A), a sum calculation section 27b for finding (V1A+V2A), and a division section 27c for dividing the output value of the difference calculation section 27a by the output value of the sum calculation section 27b. Since (V1A+V2A) found by the sum calculation section 27b is a value equivalent to all currents of the position detection element 21 (11A+12A) and corresponds to the light reception amount, the analog output value is normalized so that light reception amount change caused by the reflection factor difference on the surface of the object 3 or the laser light strength difference produced by the laser diode 11 is not affected by the value output by the operation section 27. This means that ideally the distance to the object 3 can be found even if the light reception amount changes.
By the way, an art is proposed for placing two optical displacement measurement devices, which will be hereinafter abbreviated to displacement sensors, 1A and 1B, placed so as to face each other for measuring the distance to both faces of an object 3 to measure the thickness thereof. That is, the two displacement sensors 1A and 1B are placed so that light beams are formed in opposed directions to each other in line, and the measurement values at the displacement sensors are subtracted from the distance therebetween for finding the thickness of the object 3. In fact, to the addition value of the measurement results with the displacement sensors 1A and 1B about a reference gage having a known thickness, the thickness of the reference gage is added to generate a correction constant .alpha., which is equivalent to the substantial distance between the two displacement sensors 1A and 1B. The sum of the distances LA and LB measured with the displacement sensors 1A and 1B for the object 3 to be measured is subtracted from the correction constant .alpha. (.alpha.-(LA+LB)), whereby the thickness of the object 3 can be found.
By the way, when the two displacement sensors 1A and 1B are used to measure the thickness of the object 3 as described above, if the object 3 is nontranslucent, the two displacement sensors 1A and 1B measure the thickness separately and no problem arises. If the object 3 is translucent, both the displacement sensors 1A and 1B interfere with each other and cannot measure the thickness with good accuracy. For example, when the object 3 is paper, ceramic, etc., beam light reaches the inside of the object 3. Thus, when the thickness is small, a part of the beam light leaks to the rear face and is received at the opposed displacement sensor 1A, 1B. Here, the beams are modulated as described above and normally a phase difference exists therebetween. Moreover, it is difficult to make the characteristics of the two displacement sensors 1A and 1B match completely. Thus, normally there is also a difference between frequencies fA and fB modulating the beams. Therefore, the length of light received at the displacement sensor 1A, 1B changes with time at the beat frequency equivalent to the difference between the frequencies fA and fB, as shown in FIG. 30. The time change amplitude changes with the ratio between the reflected light and transmitted light. The difference between frequencies fA and fB is caused for various reasons; it is also caused by circuit constant variations, temperature characteristic variations, etc. The received light amplitude and frequency cannot be determined uniquely.
Therefore, it is difficult to remove such a beat component and the beat component is contained in input to the operation section 27. That is, the position information signals V1A and V2A input to the operation section 27 (V1B and V2B to the operation section 27 of the displacement sensor 1B) change in amplitude at beat frequency, thus the denominator when the distance is found ((V1A-V2A), (V1B-V2B)) varies; resultantly, the measurement accuracy or resolution always changes with time.
Ideally, the distances match even if the light reception amount varies. In fact, the processing of the position detection section 21 to the operation section 27 contains a nonlinear portion. Thus, if the time change amplitude of the light reception amount is large, the found distance varies. Some of the member circuits may be saturated; distance measurement is made impossible or a large error occurs.
We have discussed the example wherein the beams of the two displacement sensors 1A and 1B interfere with each other to measure the thickness of the object 3. If a plurality of the displacement sensors 1A and 1B are provided in positional relationship such that they are placed with light beams interfering with each other, for example, such that the displacement sensors 1A and 1B are placed side by side and a light transmission spot is formed within the view field of the position detection element 21, there is also a possibility that interference may occur.